The BACTEC 460 TB System (Becton Dickinson Diagnostic Instrument Systems, Sparks, MD) is widely used to perform susceptibility testing of clinical isolates of Mycobacterium tuberculosis. This system currently provides more rapid susceptibility results than other available methodologies, and its use is currently recommended for the earliest possible detection by non-molecular methods of multidrug-resistant M. tuberculosis (MDR-TB) isolates. Preliminary studies done in our laboratory suggested that some of the organism inoculum recommendations provided by the manufacturer may not in fact be optimal and may require repetition of the test to obtain interpretable results. We evaluated the use of different inoculum sizes with this system. We found that while the higher inoculum concentration (which we suspected would generally be more useful) was in fact satisfactory, it showed no consistent superiority to the lower inoculum concentration recommended by the manufacturer. This project has been concluded.
